An outboard motor having a water-cooled engine collects water of a river or lake or seawater from the outside and uses it as an engine coolant. After the use, the coolant remaining in a water jacket of the engine is discharged.
Japanese Laid-open Patent Publication No. 8-312346 discusses an engine including a water jacket provided in an engine block, a first coolant passage connected to an upstream side of a coolant flow direction of the water jacket, a second coolant passage connected to a downstream side of the coolant flow direction of the water jacket, a coolant pump that supplies the coolant to the first coolant passage, and a thermo-valve provided in the second coolant passage, wherein a relief valve for bypassing the coolant from the first coolant passage to the second coolant passage is provided in a bypass passage that connects the first coolant passage to a downstream position of the coolant flow direction relative to the thermo-valve of the second coolant passage, and a flushing water inlet portion is provided in the second coolant passage between the water jacket and the thermo-valve.
In order to discharge the coolant remaining in the water jacket of the engine after an operation stop, it is necessary to exchange the water remaining in the water jacket with the external air. However, in the configuration discussed in Japanese Laid-open Patent Publication No. 8-312346, if a temperature of the coolant decreases after the operation stop, the thermo-valve provided in the downstream side of the water jacket is closed. Therefore, the second coolant passage is closed. For this reason, a long time is necessary to exchange the coolant remaining in the water jacket with the external air.